This invention is related to tunable surface plasmon resonance (SPR) sensors and devices and methods utilizing the phenomenon of tunable surface plasmon resonance.
The surface plasmon waves, being oscillations of free electrons in a metallic or semiconductor film or layer, may be induced through interaction of photons with the free electrons in the film or layer. A maximum transfer of energy from the photon flux to the surface plasmon wave is observed as a decrease in the optical reflectivity of the metal or semiconductor film or layer, which occurs over a narrow set of wavelengths. This phenomenon is known as the surface plasmon resonance (SPR). The layer or film of metal or semiconductor in which SPR may be established may be referred to as SPR layer or SPR film.
The conditions required for photon-induced SPR include the requirement that the photons' electric field is aligned with the electric field associated with electron oscillations; that photon momentum is matched with that of the surface plasmons; and that the real component of the dielectric constant of the SPR layer is negative while the dielectric constants of the cladding materials on both sides of the SPR layer are positive. Thus, the photon-electron interactions are dictated by the properties of the materials used and by the wavelength, polarization, and angle of the incident light.
SPR may be observed, for example, by placing a thin layer of metal between two dielectrics with different dielectric constants, for example, glass and air. When the angle between the direction orthogonal to the glass surface and the direction of an incident p-polarized electromagnetic wave in the glass is greater than a critical angle, a total internal reflection takes place, i.e. the electromagnetic wave is fully reflected back into the glass. However, for some wavelengths and angles of incidence, the incident photons, instead of being reflected, are absorbed by plasmons in the metal; in other words, the energy of the electromagnetic wave is transferred to the plasmons. At these angles and wavelengths, the surface plasmons induced at the metal/air interface reinforce the surface plasmons at the glass/metal interface. These angles and wavelengths depend on the dielectric constant and thickness of the metal layer and on the dielectric constants of the dielectrics on the both sides of the metal.
The oscillating wave of electrons propagates along the SPR interface until it either radiatively or non-radiatively decays. Plasmon-photon interactions may be used to modulate light in a linear or a nonlinear manner. Small applied electrical fields also influence the propagation of a surface plasmon wave. Other methods of establishing surface plasmon resonance in a layer of material are known in the pertinent art.
One use of this phenomenon is observing changes in the medium on one side of the metal layer. For example, when on the air side of the metal layer a layer of some substance (with a dielectric constant different from the dielectric constant of air) begins to grow, the amount of reflected light in the glass changes because the dielectric constant of a dielectric in contact with the metal has changed.
One of the techniques used to electronically tune the surface plasmon wavelength is to apply a voltage across the metal film to change the electron density in the film. That technique has limitation in that the maximum frequency at which the voltage can be adjusted is determined by the RC time constant of the metal film. Therefore, because the area of the metal film in most SPR sensors is large, the maximum frequency tends to be low. Another technique of using various electro-optical materials for tuning imposes a restriction of the range of wavelengths that can be used in such sensor.